11,492 research outputs found
Quantum evolution across singularities: the case of geometrical resolutions
We continue the study of time-dependent Hamiltonians with an isolated
singularity in their time dependence, describing propagation on singular
space-times. In previous work, two of us have proposed a "minimal subtraction"
prescription for the simplest class of such systems, involving Hamiltonians
with only one singular term. On the other hand, Hamiltonians corresponding to
geometrical resolutions of space-time tend to involve multiple operator
structures (multiple types of dependence on the canonical variables) in an
essential way.
We consider some of the general properties of such (near-)singular
Hamiltonian systems, and further specialize to the case of a free scalar field
on a two-parameter generalization of the null-brane space-time. We find that
the singular limit of free scalar field evolution exists for a discrete subset
of the possible values of the two parameters. The coordinates we introduce
reveal a peculiar reflection property of scalar field propagation on the
generalized (as well as the original) null-brane. We further present a simple
family of pp-wave geometries whose singular limit is a light-like hyperplane
(discontinuously) reflecting the positions of particles as they pass through
it.Comment: 25 pages, 1 figur
Four-Dimensional SCFTs from M5-Branes
We engineer a large new set of four-dimensional N=1 superconformal field
theories by wrapping M5-branes on complex curves. We present new supersymmetric
AdS_5 M-theory backgrounds which describe these fixed points at large N, and
then directly construct the dual four-dimensional CFTs for a certain subset of
these solutions. Additionally, we provide a direct check of the central charges
of these theories by using the M5-brane anomaly polynomial. This is a companion
paper which elaborates upon results reported in arXiv:1112:5487.Comment: 45 pages, 11 figure
Special geometry, black holes and Euclidean supersymmetry
We review recent developments in special geometry and explain its role in the
theory of supersymmetric black holes. To make this article self-contained, a
short introduction to black holes is given, with emphasis on the laws of black
hole mechanics and black hole entropy. We also summarize the existing results
on the para-complex version of special geometry, which occurs in Euclidean
supersymmetry. The role of real coordinates in special geometry is illustrated,
and we briefly indicate how Euclidean supersymmetry can be used to study
stationary black hole solutions via dimensional reduction over time.
This article is an updated and substantially extended version of the previous
review article `New developments in special geometry', hep-th/0602171.Comment: 39 pages, Contribution to the Handbook on Pseudo-Riemannian Geometry
and Supersymmtry, ed. V. Corte
New aspects of two-dimensional quantum gravity
Causal dynamical triangulations (CDT) can be used as a regularization of
quantum gravity. In two dimensions the theory can be solved anlytically, even
before the cut-off is removed and one can study in detail how to take the
continuum limit. We show how the CDT theory is related to Euclidean 2d quantum
gravity (Liouville quantum gravity), how it can be generalized and how this
generalized CDT model has a string field theory representation as well as a
matrix model representationof a new kind, and finally how it examplifies the
possibility that time in quantum gravity might be the stochastic time related
to the branching of space into baby universes.Comment: Lectures presented at the 49th Cracow School of Theoretical Physics,
"Non-Perturbative Gravity and Quantum Chromodynamics", Zakopane May 31-June
10, 2009. To appear in Acta Physica Polonica B 40 (2009) 1001-103
Supersymmetric AdS Backgrounds in String and M-theory
We first present a short review of general supersymmetric compactifications
in string and M-theory using the language of G-structures and intrinsic
torsion. We then summarize recent work on the generic conditions for
supersymmetric AdS_5 backgrounds in M-theory and the construction of classes of
new solutions. Turning to AdS_5 compactifications in type IIB, we summarize the
construction of an infinite class of new Sasaki-Einstein manifolds in dimension
2k+3 given a positive curvature Kahler-Einstein base manifold in dimension 2k.
For k=1 these describe new supergravity duals for N=1 superconformal field
theories with both rational and irrational R-charges and central charge. We
also present a generalization of this construction, that has not appeared
elsewhere in the literature, to the case where the base is a product of
Kahler-Einstein manifolds.Comment: LaTeX, 35 pages, to appear in the proceedings of the 73rd Meeting
between Physicists and Mathematicians "(A)dS/CFT correspondence", Strasbourg,
September 11-13, 200
Charge Transport in Single Au|Alkanedithiol|Au Junctions: Coordination Geometries and Conformational Degrees of Freedom
Recent STM molecular break-junction experiments have revealed multiple series
of peaks in the conductance histograms of alkanedithiols. To resolve a current
controversy, we present here an in-depth study of charge transport properties
of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM
measurements unambiguously confirm features showing more than one set of
junction configurations. Based on quantum chemistry calculations, we propose
that certain combinations of different sulfur-gold couplings and trans/gauche
conformations act as the driving agents. The present study may have
implications for experimental methodology: whenever conductances of different
junction conformations are not statistically independent, the conductance
histogram technique can exhibit a single series only, even though a much larger
abundance of microscopic realizations exists.Comment: 19 pages, 9 figures, 1 table; published versio
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